1. Field of the Invention
This invention relates to methods for producing biuret, and in particular, it relates to methods for recovering biuret from urea.
2. Description of the Art
Biuret is widely used in commerce as a precursor for pharmaceuticals, herbicides, and other compounds, as an analytical reagent, and as a ruminant feed supplement. All of these utilities benefit from (if not require) the use of relatively pure biuret.
While biuret can be produced by several chemical methods, it is typically obtained by pyrolyzing urea at a temperature of at least 130.degree. C. and for a period of time sufficient to convert at least a portion of the urea to biuret. An illustrative urea pyrolysis process is discussed by Shipley and Watchorn in British Patent No. 1,156,099. As disclosed by Shipley et al., the method produces a mixture of urea, biuret and higher molecular weight urea condensation products such as triuret, cyanuric acid, ammelide, melamine, ammonium cyanurate, methylene diurea, and/or other compounds.
Urea manufactured as solid prills is often treated at temperatures that result in some conversion of urea to biuret and, in many cases, the formation of higher molecular weight compounds as well. While the biuret concentration in prilled ureas is typically low, e.g., 0.5 to 3 weight percent, the amount of biuret contained in such products is substantial due to the large volume of prilled urea manufactured annually. Many of the commercial biuret-containing prilled ureas also contain higher molecular weight urea condensation products such as those mentioned above.
Many of the higher molecular weight condensation products present in some ureas appear to form by the reaction of urea with itself or with previously formed condensation products, or by reactions of, or between, previously formed condensation products. Others, such as methylene diurea, appear to form by the reaction of urea and/or condensation products with additives or other impurities such as formaldehyde which is sometimes employed as a urea anti-caking agent. Regardless of their origin, one or more of such impurities are known to exist in biuret obtained from urea by presently available methods as discussed by Shipley et al., supra, and Kaasenbrood in U.S. Pat. No. 3,185,731.
While urea pyrolysis and prilled urea manufacture afford an ample supply of biuret, the major utilities for biuret benefit from the use of that compound in relatively pure form. For instance, analytical procedures and pharmaceutical and herbicide manufacturing practices involving the use of biuret are most often unacceptably complicated by the presence of higher molecular weight condensation products, and the biuret dosage rate which can be employed in ruminant feed supplements is often limited by the toxicity of such impurities.
Methods presently available for commercially recovering biuret from urea typically involve low temperature crystallization procedures such as those discussed by Shipley et al. and Kaasenbrood, supra, in which the urea and/or biuret are recrystallized several times and separated, and the solid phase is washed to obtain purified urea and/or biuret. While such methods effectively separate biuret from urea, such separation is not complete, and the methods involve expensive, low temperature recrystallization procedures. A substantial amount of biuret generally remains in the urea fraction, and the biuret fraction typically contains minor amounts of urea, even after repeated recrystallization. Furthermore, such procedures do not efficiently separate biuret from urea and cogeneric impurities such as higher molecular weight urea condensation products. Typically, some or all of the higher molecular weight impurities remain in the biuret fraction.
Several authors have disclosed that biuret can be removed from urea by contacting an aqueous biuret-containing solution with the hydroxide ion form of an anion exchanger. For instance, Fuentes et al., U.S. Pat. No. 3,903,158, disclose that impurity biuret can be removed from aqueous urea solutions by ion exchange. Takahashi et al., "Determination of Biuret in Urea by Ion Exchange Resins," Soil and Plant Food, Vol. 3, No. 3, January 1958, pgs. 142-144, disclose that biuret can be separated from aqueous urea solutions and that the biuret retained on the ion exchanger can be eluted to obtain an indication of the biuret concentration in the urea feed.
While such processes are useful for purifying urea, their use for the recovery of biuret from aqueous urea solutions suffers from several disadvantages. The maximum biuret concentration which can be achieved in an ion exchanger regenerant, such as that employed by Fuentes et al. or Takahashi et al., is at most 0.5 weight percent and occurs only in the very initial stages of regeneration. The average biuret concentration in the total regenerant is typically well below 0.1 weight percent. This is because the volume of regenerant typically employed to restore the initial biuret-retaining ability of an ion exchanger, i.e., for regeneration, is so large that the biuret concentration in the total regenerant effluent is much lower than 0.1 weight percent. Almost without exception, it is preferable to completely regenerate ion exchangers to assure the greatest capability for removing the exchanged substance (in this case biuret) in subsequent cycles. Complete regeneration is more readily accomplished by the use of large regenerant volumes. In addition, the use of alkaline regenerants as disclosed by Fuentes et al. destroys biuret as disclosed in our U.S. Pat. No. 4,345,099 for Method of Selectively Removing Biuret from Urea and Compositions for Use Therein, the disclosure of which is incorporated herein by reference in its entirety.
The low biuret concentrations afforded in the exchanger regenerants of Fuentes et al. and Takahashi et al. render those solutions impractical for a variety of reasons. (In fact, the strongly alkaline regenerants of Fuentes et al. may not contain any biuret at all unless they are neutralized and/or cooled to prevent biuret hydrolysis.) The solubility of biuret in water at 0.degree. C. is 0.53 weight percent. (Urea, Its Properties and Manufacture, Chao, Chao's Institute, West Covina, Calif., Library of Congress Catalogue Card No. Ai-11524.) Thus, biuret could not be crystallized from solutions of such low biuret concentration unless the solutions were modified to substantially depress their freezing points, and such modification of the solutions might itself prevent biuret crystallization even at lower temperatures. Thus, the commercial use of such solutions would require the shipment of large volumes of water with the attendant cost of such shipment, or evaporation of enough of the water to significantly increase biuret concentration. Obviously, such evaporation adds further expense to the process.
A further disadvantage associated with the prior art methods for separating biuret from urea involves the presence of a significant proportion of higher molecular weight urea condensation products in a substantial portion of biuret-containing ureas. While the prior art crystallization processes could be employed to separate biuret from some of the higher molecular weight urea condensation products, those processes, as mentioned above, require time-consuming, repeated low temperature recrystallization. The expense involved in such methods obviously increases the cost of biuret derived from such sources and limits its application as a result. For instance, ruminant feed supplement manufacturers generally choose to use relatively impure, less expensive biuret at dosage rates which are sufficiently low to avoid the toxic effects of the higher molecular weight impurities.
Furthermore, neither Fuentes et al. nor Takahashi et al. mention the presence of materials other than urea and biuret or the possibility that impurity-free biuret can be recovered from urea solutions which contain higher molecular weight urea condensation products. In fact, Takahashi et al. observe that "usually, urea for agriculture does not contain nitrogen compounds other than biuret." While that is often the case, some urea solutions, and in particular those formed from urea which has been pyrolyzed at temperatures above 130.degree. C. for any significant period of time, contain a significant proportion of urea condensation products of higher molecular weight than biuret, some of which are toxic, and all of which can impair product utility.